In recent years, run-flat tires capable of safety driving even at puncture of a tire have been developed. These tires are provided with a thick reinforcing rubber layer having high elasticity at a sidewall part thereof and therefore keep rigidity when puncture occurs, and also rubber breakage is decreased and running a certain distance is possible even when the tire is subject to bending deformation repeatedly. Accordingly, there is no necessity of always having a spare tire ready and reduction of a whole weight of a vehicle is intended, therefore, fuel cost can be reduced. However, in running a run-flat tire at its puncture, a running speed and distance are limited, and thus it is necessary to improve durability thereof.
JP2005-264150A discloses a rubber composition which comprises specified amounts of a diene rubber, specific carbon black, specific flake-like natural ore, and sulfur or a sulfur compound, thereby having both low heat build-up property and high strength and being capable of improving run-flat durability. There is also disclosed a run-flat tire using the rubber composition for a reinforcing layer of a sidewall part. However, since a flake-like natural ore has a high specific gravity, if it is compounded in a rubber, a weight per volume becomes heavy, thus, fuel cost is increased, and further, there still remains room for improvement of run-flat durability, low heat build-up property, and strength at break.